We studied contestants from The Biggest Loser weight loss competition as well as patients who underwent gastric bypass surgery and measured changes in resting metabolic rate, and body composition over time. We compared these groups and found that the vigorous exercise performed by The Biggest Loser contestants led to preservation of lean tissue mass despite similar weight loss to patients who had bypass surgery. Both groups exhibited metabolic slowing following weight loss that was out of proportion to their body composition change. Furthermore, this metabolic adaptation was found to be correlated with the degree of energy imbalance as well as the changes in the blood levels of the hormone leptin. We recently found that the metabolic adaptations persisted in the Biggest Loser participants 6 years after the weight loss competition despite substantial average weight regain.